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Here’s what I know – I mean, I could look up all sorts of information, but this is supposed to be what your preceptor knows, right? Is your preceptor a medical physicist? No! But can your preceptor work an intra-aortic balloon pump, a CVVH machine, and a Zoll pacing box (how about one at a time, okay?) Hopefully! So: x-rays are a kind of dangerous but useful ionizing radiation. They produce images on silver-coated film that lives in the x-ray plates that we’re forever putting behind one part of our patients or another. The dangers in exposure to x-rays are two: how much power they use to shoot, and how close you are to the shot. “The exposure varies inversely with the square of the distance from the source.” Meaning: that your risk of exposure drops a whole lot when you get away from where the machine is pointing at. So stand way back. I usually stand behind the tech shooting the film (grin!).
What are some common x-ray procedures that my patients may have in the MICU? Our patients get “imaged” a lot. Most of our images are portables, shot in the bed, although all too often patients will have to travel to the radiology suites for CT or MRI studies. Some common situations: “Plain films”:
Rarely, we’ll have bone-fracture films to shoot, but usually fractures in our patients are stabilized in the most basic way by orthopedics, and then left to be resolved once the more life-threatening problems are settled. CT scans and MRI’s: (starting from the top and working south, and only listing the ones that come readily to mind)
X-ray techs shoot all our films. There are specialty techs that run the CT scanners and the MRI machines. I believe that there is a single tech that does all the portable CT scans. Don’t forget though, that on trips to the scanners you are the person in charge of the patient clinically. If you think there’s a problem or the chance of a problem – speak up! The techs are used to this, and are more than willing to help you get the patient through the scan safely. Can I stay in the room with the patient if my patient is getting x-rayed? I find that I rarely need to – the only time I can think of is if the patient is having lateral decubitus films shot (side-lying – they’re usually looking to see if a collection of fluid moves downwards with gravity and “layers out”). It can be hard to keep a patient in this position when they’re hooked up to lots of hardware – check with the tech - you may find yourself wearing lead and holding the patient up. By all means, use appropriate measures to safely, briefly sedate your patient if she needs it for the x-ray. If you’re taking your patient off the floor for CT or an MRI, check with the team – if your patient can’t be accurately scanned because of agitation, there’s no point in making the trek if you can’t safely give them sedation to help them hold still.
What are those clip things that x-ray techs wear? Should we wear them? The techs all wear film dosimeters – gadgets that measured their cumulative exposure to radiation over some given period of time. As for nurses wearing them – I need to ask around about this. (Update – the techs said no.)
It seems like my patient has been x-rayed twelve times today – is that safe? It’s obviously a question of priorities: will the patient benefit more from having the x-ray studies, or from not having them? Looking around on the web I found an interesting way of looking at the problem: you compare the amount of radiation from the x-ray study with the amount of normal “background” radiation the patient might receive just by lying still in bed, bombarded by cosmic rays, and radon from the rumpus room in the basement. They call this the “Background Equivalent Radiation Time” – or BERT. Here are some of the numbers:
The website giving this information went on to say that “no studies of radiation to humans have demonstrated an increase in cancer at the doses used in diagnostic radiology…”. I’m obviously not trying to do a comprehensive review here – but as far as I went, the information was reassuring. Your milage may vary…
Worth mentioning – he discovered that these strange rays generated by his vacuum tube could pass through certain materials, make interesting images on silver-coated photographic plate. Not knowing what the rays were or where they came from, he called them “X” - like the unknown quantity in an algebra formula. Here he is:
Is it true that Marie Curie glowed in the dark? Neat rumor, huh? My daughter did a report on Marie in high school, and says that to this day they still can’t handle her diaries – they’re too radioactive.
I have no idea, but my daughter says there’s an old joke: Pierre: (going to bed at night) Marie, turn the lights out. Marie: They are out, dear.
What is a CAT scan? What is a spiral CAT scan? How long do scans take? Nurses have a pretty good idea of what CT scans are – they produce a series of “cuts”, images across the body working upwards or downwards through the body section in question. Spiral CT’s are a newer kind of scan – the scanning tube rotates continuously as the patient moves along through the scanner – the result is better imaging with lower radiation exposure. Most scans nowadays take less than half an hour – it’s transporting your possibly unstable patient to the scanner and back that makes for all the stress. There’s a full description of how you might plan and carry out a trip to the scanner in the “New In the ICU” FAQ.
CTA stands for CT Angiography – the idea is to do a spiral CT scan while IV contrast is injected. CTA can apparently require a lot of contrast – 100-150 ml. This may be a bad thing for your patient’s kidneys…CTA seems to be the scan of choice when evaluating PE’s and vascular aneurysms of one kind or another.
What’s the difference between a CT scan and an MRI? MRI stands for Magnetic Resonance Imaging – it uses radio-frequency waves instead of ionizing radiation to generate an image. The machine involves the use of very powerful magnets – they will pull anything made of ferrous material (iron/steel) right off of you into the machine, and you will not be able to get it out until the techs shut the magnets down – this usually makes them very unhappy. There was a famous story from somewhere about a code cart getting whipped entirely up off the floor… Likewise, taking a patient with implanted objects can be very dangerous – how about pacemakers? Hip replacements? Cerebral aneurysm clips? Think about this every time you take a patient to the MRI suite – check with the team, and check with the scanner techs to make sure the scan is safe. MRI scans take much longer than CT’s – get orders for appropriate sedation (I find a little propofol in my coffee is very helpful – ow! Oh, you meant the patient!) before you go. Here are a couple of nice images to show the difference in quality:
MRA is “Magnetic Resonance Angiography” – which is to say, MRI looking at blood vessel flow, probably using contrast. MRI studies use a contrast material called “gadolinium” - you’ll hear the techs say things like: “With or without gado?” Gadolinium turns out to be an element – here’s what I could find out about it: “Gadolinium, chelated to a carrier molecule, is an intravenously injected MR contrast agent which …normally stays in blood vessels…it has the effect of making vessels, vascular tissues, and areas of blood leakage appear brighter.” (Thanks Ray Hsu, Washington U. School of Medicine!) So this is what you’ll probably see them give when you’re looking for a bleed somewhere… “Gadolinium is excreted through the kidneys, with a half –life of 1.25 – 1.6 hours.” Gado has the reputation of being very low on the allergic reaction list.
Why do some CT tests use contrast? They help light up the structures that you’re trying to see. In CT scanning, the contrast dye is iodine-based – which is why patients with allergies to shellfish aren’t supposed to get them. These dyes definitely have dangers associated with them: obviously, some people are going to have severe allergic reactions. The other problem, and we see this one more often than we’d like to, is the fact that a dye load can really, seriously hurt a patient’s kidney function, especially if they’ve got some degree of renal failure already. Here are some of the main points:
What is the connection between iodine-based contrast and renal failure? Here I’m going to summarize one of a really neat series of clinical pearls from the US Army Pharmacy website, edited by Major Dave Andersen. This one was comprehensive yet succinct, and extremely clear. Thanks, Major Andersen. A 62-year old patient with diabetic nephropathy is booked for a CT scan with contrast. All labs are normal except for a glucose of 135, and a creatinine of 2.4. (Uh-oh…I’ve been in too many of these situations myself. Can you spell CVVH?) The radiologist is concerned about giving contrast to a patient with a creatinine over 2.0. Is there anything that can prevent or minimize further kidney damage? Acute renal failure from IV contrast – this they define as a rise in creatinine of more than 0.5 within 48 hours after the dose – ranges from 9-40% in diabetics with mild-to-moderate renal insufficiency, to 50-90% in diabetics with severe chronic renal insufficiency. (Ack! I take my glucophage, don’t I? And the doc says my feet tingle because I stand up all night…) Some summary points:
What is this I hear about mucomyst? A recent study (NEJM 2000 43: 180-4) showed that a 600mg dose of Mucomyst (acetylcysteine) on the day before and the day after the contrast dose significantly lowered the incidence of contrast-induced acute renal failure. Anybody know how this works?
Do we give contrast in the MICU? We give oral contrast in the form of gastrografin. The CT orders have built-in dosing orders to tell you what to do – usually it’s something like 7.5cc of gastrografin in 200cc of water either orally (ack!) or through an NG tube, repeated several times. Check with the team if you’re worried about your patient’s kidneys.
What kind of IV access does my patient have to have to get IV contrast? We take patients with all sorts of IV access to the scanners, but for some reason the techs down there want the patient to have a plain, garden-variety heplock in one arm or the other. Anybody know why they don’t use a central line? Make sure the IV is patent, and in a sizable vein – that contrast gets injected pretty fast…
Apparently this stuff is very safe to use. It is iodine based.
What is the problem with Glucophage (metformin)? (I took a personal interest in this one…) Glucophage has the rare but unhappy ability of provoking a severe lactic acidosis, especially in renal failure situations. If the IV contrast dose were to push a patient from, maybe, CRI to ARF, then the presence of glucophage in that situation would be a bad thing. It appears that the routine is to hold glucophage for a day before the exam and for two days afterwards… good to know. Images
Chest film with a really clear trachea and carina.
Endotracheal tube in the right main stem.
ET tube pulled back to the proper position.
Chest film with ETT, CVP line, and maybe an NG tube.
Chest film with a trach in place, and old sternotomy wire sutures.
An NGT causing a pneumo. Looks like a Dobhoff.
Pretty big effusion over there on the patient’s right. What should we do?
A chest film with two chest tubes: one’s in position, the other isn’t.
A PA line with an interesting object nearby.
An IABP tip. (Really?), and a PA line, probably not in far enough.
An abdominal film with dilated bowel loops.
One patient, two films, before and after developing tamponade.
Who is this, and what happened to him?
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